Depending on coaster manufacturer (I'm thinking Intamin or Premier), level of detail, and ride technology for Gringotts, that will likely be in the neighborhood of 80-120 million alone. Assuming this is the only major attraction, I'd guess 400 million. Diagon Alley won't be as grand as Hogsmeade/Hogwarts and this is a floor-to-ceiling job, whereas WWOHP1 was able to re-imagine previous attractions.
To spark further conversation, what kind of system do you think Gringotts will use? Launch, chain lift, elevator lift, something else?